1. Field of the Invention
This invention relates to a device for detecting abnormality of spark plugs for internal combustion engines and a misfire-detecting system therefor incorporating the device.
2. Prior Art
In an internal combustion engine having spark plugs, a misfire can occur, in which normal ignition does not take place at one or more of the spark plugs. Misfires are largely classified into ones attributable to the fuel supply system and ones attributable to the ignition system. Misfires attributable to the fuel supply system are caused by the supply of a lean mixture or a rich mixture to the engine, while misfires attributable to the ignition system are caused by failure to spark (so-called mis-sparking), i.e. normal spark discharge does not take place at the spark plug, for example, due to smoking or wetting of the spark plug with fuel, particularly adhesion of carbon in the fuel or unburnt fuel to the spark plug, or abnormality in the sparking voltage supply system.
The present assignee has already proposed a misfire-detecting system for detecting misfires attributable to the fuel supply system, which comprises sparking voltage detecting means which detects sparking voltage, i.e. voltage across electrodes of the spark plug, and misfire-determining means which determines that a misfire has occurred based on a detected value of the sparking voltage, e.g. when a time period over which the detected value of the sparking voltage exceeds a predetermined reference value (Japanese Patent Application No. 3-326507 and corresponding U.S. Pat. No. 5,215,067).
On the other hand, in detecting misfires attributable to the ignition system, e.g. in detecting smoking of a spark plug, a measuring instrument or the like is conventionally used to directly measure resistance between the electrodes of the spark plug.
As stated above, when the performance a spark plug per se is degraded due to smoking thereof, etc. there is a possibility that normal spark discharge does not take place at the spark plug. However, the misfire-detecting system disclosed by the above-mentioned publications does not take this problem into consideration, and hence there remains an inconvenience to be eliminated for the purpose of enhancing the accuracy of misfire-detection. More specifically, when the combustion of the engine is unstable e.g. due to a low engine temperature, an unburnt fuel component (carbon) is deposited between the electrodes of a spark plug, i.e. between a central electrode and a grounding electrode. If carbon is deposited in a large amount, current supplied from the ignition coil to the central electrode eventually flows from the central electrode to the grounding electrode via the carbon deposited therebetween. Under such a condition of the spark plug, normal spark discharge does not take place, resulting in a misfire.
Therefore, it is necessary to detect the conditions of spark plugs per se, e.g. smoking thereof. However, it has been difficult to install a device for detecting the conditions of the spark plugs per se on an automotive vehicle, and therefore the resistance between the electrodes of the spark plugs has been directly measured by the use of a measuring instrument or the like as stated above. Thus, a misfire-detecting system for internal combustion engines which can detect a misfire with the conditions of spark plugs taken into account has not been realized yet.